In accordance with our U.S. patent application Ser. No. 743,323, filed Nov. 19, 1976, now U.S. Pat. No. 4,116,788, it is possible to produce suitable masses based on acrylic ester compounds which can be polymerized by ultraviolet light and which contain mono- and/or polyfunctional acrylic or methacrylic ester compounds by the use of benzoin or benzoin derivatives acting as ultraviolet initiators, possibly also containing stabilizers and fillers, where aliphatic, aromatic or aliphatic-aromatic phosphites in a quantity ranging from 0.1 to 20 percent by weight, based on the acrylic ester compounds are present as activators for the ultraviolet polymerization. The phosphites are preferably used in these masses in a concentration of 0.1 to 2, and especially 0.2 to 1 percent by weight. The presence of the phosphites in concentrations as specified results in a strong activation of the initiating effect of the benzoin or its derivatives on the polymerization of the acrylic ester compounds by ultraviolet light.
It was found that this activation by the phosphites will occur also if monoketals of aromatic 1,2-diketone compounds, acting as ultraviolet initiators of the polymerization, are used. These monoacetals or -ketals of the 1,2-diketone compounds are per se known to act as initiators of the ultraviolet polymerization of various polymerizable masses. The published German application No. 22 32 365 characterizes the monoketals, especially of the aromatic diketones, such as benzil, as photo-initiators for the ultraviolet hardening of polyester resins. However, if these initiators are used, the radiation times needed for the hardening are quite extensive. In order to attain proper hardenings, radiation times lasting several minutes are required by the process disclosed by this German Offenlegungschrift No. 22 32 365.
The published German application No. 23 37 813 describes in detail a particularly suitable method for the production of these monoketals of aromatic 1,2-diketones and furthermore characterizes these compounds as means to sensitize the photopolymerization of polymerizable systems.
In accordance with the present invention, there is provided a photopolymerizable composition consisting essentially of acrylic or methacrylic acid esters which can be polymerized by ultraviolet light and which contains mono- or polyfunctional acrylic or methacrylic acid ester compounds and as ultraviolet initiators a monoketal of an aromatic 1,2-diketone compound and from about 0.1 to 20 percent by weight of the photopolymerizable ester compounds of an aliphatic, aromatic or mixed aliphatic-aromatic organic phosphite compound. Preferably, the monoketals are present in amounts of from about 0.05 to 20, most preferably from about 0.1 to 2, percent by weight of the photopolymerizable ester compounds.
The admixture of the aliphatic, aromatic, or mixed aliphatic-aromatic phosphites in a concentration of 0.1 to 20, and preferably 0.2 to 2.0, percent by weight of the photopolymerizable ester compounds and the monoketals of aromatic diketones, acting as ultraviolet sensitizers, as proposed by the present invention, will result in an activation of the polymerization which, in comparison with the polymerization attainable solely by the monoketals of the diketones, leads ordinarily to a two to five-fold acceleration. This greatly accelerating effect of the phosphites is particularly unexpected and surprising because it is stated in German Offenlegungschrifts Nos. 19 34 637 and 23 65 497, which are also concerned with polymerizable masses and the use of benzoin derivatives or monoketals of aromatic 1,2-diketones as photoinitiators, that phosphites, at a very weak concentration between 200 and 800 ppm, and also copper naphthenate or other phosphorus compounds such as triphenylphosphine or tributylphosphine, can be utilized in connection with this process as stabilizers to prevent an undesirable polymerization and for the purpose of increasing the stability during storage in the dark. In view of these circumstances a person skilled in the art had to assume that in the case of a polymerization system, containing monomers based on an acrylic acid and monoketals of the diketones as ultraviolet initiators, that these phosphites would have the effect of retarders rather than of activators of these systems.